(1.) Field of the Invention
The present invention relates to a vehicle lamp such as a tail lamp, a turn signal lamp and a stop lamp and, more particularly, to a lamp having an inner lens for converting light beams from a light source to parallel light beams.
(2.) Description of the Prior Art
In general, most vehicle lamps such as tail lamps, turn signal lamps and stop lamps excluding head lamps do not have a parabolic reflector as in a head lamp since their mounting positions and spaces are limited and diverging light beams are required. In such a conventional lamp without a parabolic reflector, as shown in FIG. 1, as soon as light beams a and b from a bulb 2 pass through a lens 1, the light beams a and b are emitted as diverging light beams a' and b' outside the lamp. However, the light beams a and b are incident along diverging directions (i.e., along directions inclined with respect to the lens surface). These beams are diverged by the lens 1, so that the divergence of light beams passing through the lens 1 becomes great. The illuminance of the lamp as a whole is lowered, thereby providing only poor lighting performance.
In order to solve this problem, a Fresnel cut inner lens is used in a conventional vehicle lamp. As shown in FIG. 2, an inner lens 3 is disposed between the lens 1 and the bulb 2. The light beams a and b from the bulb 2 are converted to parallel light beams a" and b" which are directed to the lens 1. These parallel light beams a" and b" are diverged by the lens 1, thereby eliminating wasteful light components and hence improving lighting performance.
A typical conventional vehicle lamp having an inner lens of this type is illustrated in FIG. 3. Referring to FIG. 3, reference numeral 1 denotes a lens, i.e., an outer lens; 2, a bulb; and 3, an inner lens. The outer lens 1, the bulb 2 and the inner lens 3 are mounted in a housing 4 of the lamp. The housing 4 is mounted in a vehicle body 5. The outer and the inner lenses 1 and 3 are made of acrylic resin. A distance B between the inner lens 3 and the bulb 2 (i.e., a filament F) is generally 30 mm or more in consideration of heat resistance of the inner lens. A distance A between the inner and outer lenses 3 and 1 is generally 10 mm or more. This is because the pitch of the Fresnel cut inner lens 3 is excluded from visibility since the pitch is about 0.5 mm. It should be noted that the inner lens 3 is subjected to Fresnel cutting which is performed by a molding means. In consideration of molding precision, the inner lens 3 comprises a plate having a basic thickness of about 2 mm.
In this conventional vehicle lamp, an axial length of a lamp having a plurality of illumination functions such as a combination lamp is increased. When such a combination lamp is mounted in the vehicle body, an axial length L of the lamp as a whole and an axial length L.sub.1 of the lamp portion extending inside the vehicle body are relatively long. The mounting space of the vehicle body must be relatively large, thereby restricting versatility of vehicle body pressing. The conventional vehicle lamp cannot be made compact.
In addition to this disadvantage, the parallel beams directing toward the outer lens 1 through the inner lens 3 are not present in the peripheral portion of the outer lens 1. The effective illumination surface of the outer lens is decreased to degrade the lamp functions.